hMailServer is a free, open source e-mail server for Microsoft Windows. It supports all the common e-mail protocols (IMAP, SMTP, POP3) and comes with an easy-to-use COM library that can be used for integration with other software. It also has supports for virtual domains, distribution lists, antivirus, antispam, aliases, distributed domains and much more. E-mail data is stored in a database server, MySQL or MS SQL, depending on your choice.
The hMailServer installation contains a minimal MySQL-installation, so if you don't already have a database server in your network, MySQL is installed automatically when you install hMailServer.